Electronic and Vibrational States of Point Defects in Semiconductors.

The major system studied here is GaP containing Zn and O impurities. The Zn and O ions experience an attractive Coulomb interaction, so that they tend to occupy lattice sites which are near to each other, forming defect pairs. The energy of luminescence emitted from a (Zn, O) pair depends on the separation of the impurities. Thus, a luminescence spectrum contains information about the number of pairs of each possible separation. I have used this phenomenon to monitor the relative positions of Zn and O impurities in the lattice. I have observed reactions in which the impurity atoms move through the lattice under the influence of laser excitation. Specifically, I observe the dissociation of nearest-neighbor (Zn,O) pairs, and the subsequent formation of further separated pairs. The dissociation of the nearest -neighbor pairs can occur thermally, or by a photoinduced mechanism. At temperatures near 200 C, the intensity of the (Zn,O) luminescence spectra changes with time, a direct observation of the photoinduced reactions in progress. The (Zn,O) pairs are observed to dissociate by purely thermal means at temperatures near 900 C. From the rates of these two types of reactions, I identify the photoinduced pair dissociation as being a "recombination-enhanced defect reaction". In the reaction, electron-hole recombination puts the defect into a highly excited vibrational state, leading to the dissociation. This is the first observation of this sort of reaction in a system with known defect types. Thus, my study provides unique information about the electron-phonon interaction at defects. This study also has some practical application. The material GaP:(Zn,O) is used for fabricating red light-emitting-diodes, and the dissociation of the pairs provides an explanation for the degradation of these diodes. Presumably the degradation of some other semiconductor devices proceeds by mechanisms similar to those observed here.